Digital cameras and the like often adopt an autofocus (to be referred to as AF hereinafter) method (see Japanese Patent Laid-Open No. 2000-152064) called a TV-AF method. In this method, an object distance is calculated from the values of AF evaluation signals at points by moving the focus position within a given range. The AF evaluation signal is calculated using a BPF (Band Pass Filter) so as to increase as the lens is focused much more.
For example, for a distance measurement range of infinity to 50 cm, an AF evaluation signal at a focus position where the lens is focused to infinity is acquired, as shown in FIG. 6. While the focus position is sequentially moved close up to 50 cm, an AF evaluation signal at each distance is acquired. AF evaluation signals various distances are compared, and the focus position is moved to distance A where the lens is determined to be focused best, thereby controlling the focus. The abscissa in FIG. 6 represents the object distance at which the focus lens is in focus.
In general, it is difficult to successively acquire AF evaluation signals while changing the focus position. In many cases, AF evaluation signals are acquired by sampling at, e.g., a distance interval corresponding to the depth of field.
In, however, a camera having an optical system whose depth of field is shallow or a camera having a wide distance measurement range in the distance direction, even if AF evaluation signals are acquired by sampling in the above-described way, the total number of acquired data is large, resulting in a long distance measurement time.